I think if you're planning to make molds from it, and you like working with putty, it should be okay... but it may depend on what type of putty. But whatever you choose, I would stick to one material, because if you use multiple materials then it becomes very difficult to sand evenly.
Your 4th helmet?? Wow, you're pretty dedicated! I'd post a thread anyway, if I were you, as the more people who see it, the more people can post helpful advice. Although I created the 3D helmet for pepakura, I never finished building it myself, so my advice on the topic is limited. My plan was to use the pepakura design as a base to create a template from (like Alan Sinclair - Wizard of Flight - did with his Boba Fett helmet template) but I never had the time to finish it... if I ever get the time to do it, the plan is to make an 85% scale helmet for my son.